A wide-lens comprising an imaging lens system including a front lens group, and a rear lens group arranged from an object side toward an image side, and an aperture arranged therebetween, and the imaging lens system having 6 lens elements in the front and the rear lens groups in total is known (see Japanese Patent Publication No. 2002-72085).
Moreover, as a wide-angle lens which has improved the one disclosed in Japanese Patent Publication No. 2002-72085, a wide-lens comprising an imaging lens system including less than or equal to 5 lens elements in the front and the rear lens groups in total is also known. In this wide-angle lens, an angle of view is equal to or more than 190 degrees (see Japanese Patent Publication No. 2007-25499).
Because the wide-angle lens disclosed in Japanese Patent Publication No. 2002-72085 corrects various aberrations by use of many lens elements, the degree of freedom in designing is comparatively large; however, it is difficult to reduce the number of lens elements. Accordingly, the overall length of the imaging lens system becomes long, and it is difficult to achieve miniaturization of the lens, thus causing an increase in cost and weight.
On the other hand, because the wide-angle lens disclosed in Japanese Patent Publication No. 2007-25499 is a lens element disposed behind the aperture and constructed of only one lens element having positive power, an incident angle of a chief ray incident on an image plane at a position where an image height is high becomes large, that is, an angle of a chief ray of a maximum image height passing through the aperture is less than or equal to 40° (degrees). Therefore, when a camera module is constructed by arranging an image sensor at the position on the image plane, the incident angle of the chief ray incident on the image sensor at the position where the image height is high becomes larger, and the light amount incident on the image sensor decreases.
In an image sensor, or an image sensor with a microlens, the larger an incident angle of a chief ray of luminous flux incident on the imaging plane becomes, the less the luminous flux becomes that can be led to a pixel (light receiving part) on the imaging plane of the image sensor; therefore a light amount loss is generated.
Moreover, the larger an incident angle of the chief ray of luminous flux incident on the imaging plane becomes, the darker an imaging lens system with the larger an f-number becomes. Therefore, a photographic image at low illumination becomes dark for the imaging apparatus using this imaging lens system.